AWS ElastiCache: lock the remaining parameter-group ops #1690

Goal: run the rest of the parameter-group control plane under the mock's mu, the way #1472 PR0 does for creates and for parameter-group delete.

• Ops: CreateCacheParameterGroup, ModifyCacheParameterGroup, ResetCacheParameterGroup.
• Where: providers/aws/elasticache/parameters.go.
• Why: each one checks and then sets without a lock, so a concurrent duplicate create, or a modify racing a reset, can interleave.

Follow-up from #1472.